Electronic circuits include conductors and semiconductors that can be damaged when exposed to too much electrical current. When a current passes through a conductor that is too small for the current, the conductor may be heated and can be damaged or destroyed. The intense heat can melt or vaporize parts of the circuit and cause the circuit to immediately fail, or may leave latent damage that will cause the circuit to fail in the future. A very common cause of this type of damage is electrostatic discharge (ESD), due for example to static electricity, which can be generated during normal movement of a person carrying an electronic device.
Electronic devices are typically designed to protect circuits from ESD by enclosing them in insulating housings. Any electrical conductors that are exposed to the user, such as those in headphone jacks, universal serial bus (USB) connectors, memory slots, etc, may be protected by buffers that are relatively robust and resistant to ESD. ESD protection circuits may also be applied to electrical conductors that are exposed to the user.
Some electronic circuits that may be exposed to the user, however, are more difficult to protect using traditional ESD protection circuits without degrading normal function. For example, some cellular telephones include frequency modulation (FM) transmitters having a relatively large antenna located on the battery cover. The battery cover is removed from the cellular telephone each time the battery is accessed, exposing an FM transmitter connector pin that is normally connected to the FM antenna on the battery cover. If static electricity reaches the FM transmitter pin, either by contact with the user or by an electrical strike through the air, the FM transmitter could easily be damaged or destroyed. Standards bodies such as the International Electrotechnical Commission (IEC) specify stringent test requirements for electronic devices which attempt to ensure that the devices are robust enough for normal use. In one such test, the FM transmitter pin is required to withstand an 8 kV contact ESD strike and a 15 kV air ESD strike. Traditional ESD protection circuits applied to the FM transmitter pin that can withstand these ESD levels would degrade normal function of the FM transmitter. For example, because the FM transmitter is designed to operate with a high Q inductive antenna, resistive or lossy components used in a traditional ESD protection circuit would cause a severe power penalty. The characteristic impedance of the FM transmitter and antenna must also be carefully controlled based on the target FM frequencies, and resistive or lossy components may impact characteristic impedances as well as direct power losses.